Many internet users have wondered: Why in the world am I still chased around Facebook and the internet with ads for a pair of sneakers I purchased weeks ago? It would seem all sides lose in that all-too-familiar scenario. The sneaker-purchaser gets annoyed; the advertiser is trying to sell something someone already bought. In this edition of unsolved mysteries, the author examines why this still happens — and why we can’t tell the ad to stop following us.